A worker at the Goodyear tire factory on Monday guards the human resources chief, Bernard Glesser, left, and the firm's production manager Michel Dheilly, right, who have been blocked from leaving the plant in Amiens, France.

By Alexander Smith, NBC News contributor

Two managers at U.S. tire giant Goodyear were freed Tuesday after about 100 angry workers barricaded them inside a factory overnight in France.

Union-backed employees used large farm tires to barricade site director Michel Dheilly and human resources manager Bernard Glesser in a room following a meeting over lay-off pay.

Police arrived at the plant in Amiens and freed Dheilly and Glesser just before 4 p.m. local time (10 a.m. ET). Workers, who had detained the men for close to 30 hours, did not attempt to stop the officers.

Minutes after a car ferried the bosses through a large crowd gathered outside the factory, workers set fire to 100,000-euro of high-end tires which were being stored at the plant. A column of thick black smoke hung over the building for over an hour after the bosses left the site.

A Goodyear representative, speaking on condition of anonymity, told NBC News before the bosses were freed: "The situation is quite agitated.

"They have been held overnight. They have food and water but things are not easy for them."

Goodyear has been trying for years to close or sell the plant, in Amiens. The so-called "boss-napping" incident is the latest flare-up in a fractious relationship between the company and staff at the plant.

The Ohio-based firm had refused to negotiate with the General Confederation of Labour (GCT) union, which is organizing the detention, until its men were released. The company said it was preparing a statement following their release.

The Goodyear representative said there had been heated verbal exchanges inside the room. When asked if there had been physical exchanges he declined to comment.

Union chiefs from CGT are demanding lay-off pay to be increased for the 1,250 employees set to lose their jobs at the plant, according to Goodyear. The tire firm said CGT wants the packages offered to employees increased from 20,000 euro to 80,000 euro (from $27,000 to $108,000).

Goodyear said the 80,000 euro figure was the same as a lay-off package it offered employees as part of restructuring in 2012, which CGT refused.

In an official statement on Tuesday, Goodyear called for "respect of the law" and said that it "strongly condemns any form of violence." It described this type of action as "always wrong."

The Associated Press reported a statement issued by CGT: "We just want to continue to work and not swell the ranks of the unemployed and marginalized, and if for that we have to resort to extreme methods, we won't hesitate to do that."

So-called "boss-napping" became rampant in France in 2009 at the height of the economic meltdown, although the practice has since tapered off. It is punishable by five years in prison and a 75,000 euro ($102,000) fine if the boss goes free in under a week, according to the AP.

The Amiens plant, 60 miles north of the French capital, Paris, has endured a tumultuous relationship between bosses and workers ever since Goodyear announced restructuring plans some years ago.

Goodyear said the CGT union rejected an offer of a voluntary-redundancies package in 2012. It subsequently decided to close the plant in January last year.

The plant hit the headlines again when Maurice Taylor, the CEO of Illinois-based firm Titan tires which had considered taking over Goodyear, wrote an explosive email to a French government official.

"The French workforce gets paid high wages but works only three hours," he wrote, according to the AP. "They get one hour for breaks and lunch, talk for three and work for three," he said.